Add Multilib selection machinery

This patch improves the support for picking Multilibs from gcc installations.
It also provides a better approximation for the flags '-print-multi-directory'
and '-print-multi-lib'.

This reverts r201203 (i.e. re-applying r201202 with small fixes in
unittests/CMakeLists.txtto make the build bots happy).

review: http://llvm-reviews.chandlerc.com/D2538